Activated carbon is in wide use today and is currently produced from a number of different sources like coal, coconut-shell, wood, etc. Of these, coconut-shell based activated carbon has found extensive use in high-end applications like electric double layer capacitors (EDLC) due to its higher density, high specific surface area, and suitable pore size distributions. Typical coconut-shell based activated carbon in commercial use today for EDLCs has a specific surface area around 1600 m2/gm, densities in the 0.4 gm/cc range, pore sizes of <1 nanometer and pore volumes around 0.7 cm3/gm (e.g. for the YP-50 brand). Also, when naturally occurring substances like coconut-shell and coal are used as starting materials for activated carbon manufacturing, there is typically little control over the purity of the starting materials. This results in substantial purification efforts being needed during manufacturing, to bring impurity levels of commonly occurring elements like Fe, S, Cl, etc., —that have deleterious effects on specialized applications like EDLCs—to acceptable levels (<50 ppm). Recently, we have described a process to synthesize activated carbon from pure starting materials like furfuryl alcohol and acetylfuran (US patent publication US20150030525). These synthetic materials are inherently purer than naturally occurring coconut-shell char and are thus attractive starting materials for these high-end applications, when processed into nano-porous, high-surface area activated carbon.
Several approaches to manufacturing nano-porous activated carbon from synthetic sources have been described using the templating approach. In one embodiment that used an external or so-called ‘hard template’ (U.S. Pat. No. 7,887,771), a fine powder of silica was dispersed within a precursor organic compound, which was then polymerized in the presence of other catalysts and cross linking agents to form a solid, from which the template was etched away leaving behind the porous carbon. In another embodiment, metal carbides were used, from which the metal ions were etched away leaving behind a porous carbon structure (US patent application publication US20090213529 and US patent application publication US20120148473). Other examples include polymerizing different starting materials in the presence of templates, which were then removed by spray pyrolysis methods (U.S. Pat. No. 8,057,774). While these techniques allow for control of pore size in the final carbon to varying degrees—depending on the size and distribution of the original templating particles—they require a much more involved and expensive manufacturing procedure compared to coconut-shell activated carbon, which is still in widespread use.
In our recently described method for manufacturing nano-porous activated carbon we polymerized furfuryl-functional-group containing organic compounds (e.g. Furfuryl Alcohol, Furfural, Acetylfuran, etc.) in the presence of strongly acidic inorganic catalysts like silane, TiCl4 etc., (US patent application publication US20150030525). After solidification and carbonization, the catalysts ended up leaving a residue of Si or Ti-containing materials that had to be etched away. Following this, further processed using industry-standard activation techniques resulted in the necessary pore sizes (<1 nm) and surfaces areas suitable for EDLC applications. In another method [EPO publication WO2015058113], we describe the use of an external template (e.g. alumina), which itself has acidic properties, to polymerize the same furfuryl-functional-group containing compounds. In that case, no other catalysts or cross-linking agents are required. Similar to the previous embodiment, this produced a carbonaceous material that could be further processed using standard industry practice (e.g. CO2 or steam activation) to create the nano-porous carbon, after the external template of alumina was etched away.
Both these methods involving polymerization of furfuryl alcohol have some advantages over the existing coconut-shell process (e.g. purer starting materials) and the templating approach (e.g. simpler manufacturing process due to fewer starting materials). But etching is still required. Thus, improvements are desired and methods using the same starting materials, but with even fewer manufacturing steps, are attractive. There is a need to find a more efficient method of making nano-porous carbon.